This invention relates to a microwave coupling system and more particularly to a system by which a symmetrical and two orthogonal asymmetrical modes can be excited or received.
Antenna feed systems capable of generating and receiving microwave power in a plurality of modes have been developed and are known as multimode feed systems. Such multimode feed systems are often used in monopulse tracking antennas wherein the energy transmitted and received by the feed systems is combined in such a manner that sum (symmetrical) and difference mode (asymmetrical) radiation patterns are produced during transmission and/or reception. These patterns are analyzed to determine the position of a passive (reflecting) or active (radiating) object which may be either an aircraft, a missile, or a satellite or celestial body or to provide automatic tracking of these objects. Monopulse tracking systems are discussed for instance in, "Radar Handbook," by Merrill I. Skolnick, published 1970 by McGraw-Hill Book Co. and "Introduction to Monopulse," by D. R. Rhodes, published in 1959 by McGraw-Hill Book Co.
The typical tracking feed system may include several horns or apertures. When only a small number of horns are used, such as in the four-horn antennas, the radiation patterns have undesirable characteristics mainly in the form of high level sidelobes and internal losses which lower the efficiency (tracking slope) and increase the noise temperature of the system. Some prior art single aperture monopulse couplers although operative and possessing improved tracking slope, have lower than ideal gain to noise temperature ratio for their sum mode when they are used as feed systems for reflector-type antennas and when operated over a wide range of frequencies. For more details on a single aperture monopulse coupler, see pages 21-18 through 21-25 in the previously-cited "Radar Handbook."
One type of multimode coupler by which sum and difference modes can be launched into the throat of a single aperture horn is described in applicant's U.S. Pat. No. 3,560,976. It is desirable in certain applications such as in frequency reuse systems that higher gain over noise temperature (loss) ratios and particularly lower cross-polarization levels for the associated sum mode operation be provided. In frequency spectrum reuse applications for communication systems, the same frequency spectrum is reused but is communicated at orthogonal polarization. In such systems the total information carrying capacity of the system is improved by increasing the isolation between the two approximately orthogonal polarizations. This isolation, or level difference, between the two polarizations is usually maximum in the direction represented by the symmetry axis of the main beam. It is therefore highly desirable to achieve an accurate alignment of the antenna axis toward the other terminal of the link satellite by a high quality orthogonal difference mode to permit tracking. This however has to be done with minimum noise temperature (loss) contribution from the tracking circuit to the communication circuit and by minimum depolarization effect from the tracking circuit itself to the sum channel circuit.